1. Field of the Invention
The present invention relates to a negative type resist for a resist layer which is formed on the surface of a layer for etching that is formed on a substrate and a method of forming a resist pattern for forming a resist pattern on the surface of the layer for etching by using the negative type resist.
2. Discussion of Background
At present, in manufacturing methods of large scale integrated circuits (LSI) represented by a 4M or 16M dynamic random access memory (DRAM), it is very important for performing a miniaturization, to form a resist pattern which is formed on the surface of a layer for etching, for instance, of an insulating layer or a wiring layer.
It is a general practice in forming a resist pattern that after coating a positive type photoresist comprising a novolak resin and naphthoquinonediazide on the surface of a layer for etching, g line beam (wavelength 436 nm) from a mercury lamp is selectively irradiated on the positive type photoresist layer and the layer is successively developed.
In recent times, in the dynamic random access memory, the integration degree thereof has been increased further to 16M or 64M.
When the integration degree is enhanced and the miniaturization process is promoted, i line beam (wavelength 365 nm) has been employed as a beam from a beam source for selectively irradiating to a resist layer.
When the integration degree of the large scale integrated circuit is enhanced and a resist pattern of a half .mu.m or less is required from now on, it is difficult to manufacture stably the circuit when the resist pattern is formed by using the i line beam. Therefore, researches are beginning to carry out wherein a KrF excimer laser beam (wavelength 248 nm) is employed as a beam source having a shorter wavelength.
However, when a resist pattern is formed by selectively irradiating the KrF excimer laser beam to a positive type photoresist layer and by developing the layer, after coating the positive type photoresist comprising a novolak resin and naphthoquinonediazide on the surface of a layer for etching which has been employed for the g line beam or the i line beam from a mercury lamp, side walls of the pattern having faces orthogonal to the surface of the layer for etching can not be provided and a resist pattern having a high resolution can not be provided, since beam absorption is considerable in the positive type resist.
Accordingly, the following two resists have been proposed as the resist for the KrF excimer laser beam.
The first one is a chemically amplified positive type resist having two components comprising a base resin and an acid generating agent which generates an acid by receiving a light beam or three components comprising the above two components added with a dissolution restraining agent, which is easy to dissolve in an alkaline developer when an acid from the acid generating agent accelerates a polarity change reaction in a dissolution restraining protection group of the base resin or the dissolution restraining agent by performing a baking operation.
The second one is a chemically amplified negative type resist comprising a base resin, a crosslinking agent and an acid generating agent which generates an acid by receiving a light beam, which is hardened when the acid from the acid generating agent accelerates a crosslinking reaction between the base resin and the crosslinking agent by performing a baking operation.
However, in the above chemically amplified positive type resist, the acid from the acid generating agent which has generated by receiving a light beam is neutralized by a deactivating substance in environment (the acid is evaporated from the surface layer of the resist layer into the environment), by which the ratio of the polarity change reaction through the baking operation before developing is decreased, the surface layer of the resist layer is difficult to dissolve in the developer. Thereby, a phenomenon (T-Top) is caused wherein eaves are formed on the head portion of the resist pattern or a phenomenon (Skin) is caused wherein the head portions of the contiguous portions of the resist pattern are in connection with each other. Further, the acid from the acid generating agent which has been generated by receiving a light beam is neutralized by a deactivating substance on the surface of a layer for etching (the acid is diffused from portions of the resist layer contacting the layer for etching to the layer for etching), by which the ratio of the polarity change reaction is decreased through the baking operation before developing, the portions of the resist layer contacting the layer for etching are difficult to dissolve into the developer, and as a result, trails or residues are caused in the resist pattern. Such phenomena of the resist pattern are problematic in the etching operation for the layer for etching, and therefore, a resist pattern having a high resolution is difficult to obtain.
Further, also in the chemically amplified negative type resist, the acid from the acid generating agent which has generated by receiving a light beam is neutralized by a deactivating substance in environment (the deactivating substance is diffused from the environment to the surface layer of the resist layer) by which the ratio of the crosslinking reaction is decreased through the baking operation before developing, the surface layer of the resist layer is difficult to harden, and the head portions of the resist pattern are provided with a rounded shape. Further, the acid from the acid generating agent which has been generated by receiving a light beam is neutralized by a deactivating substance on the surface of a layer for etching (the deactivating substance is diffused from the layer for etching to the portions of the resist layer contacting the layer for etching) by which the ratio of the crosslinking reaction is decreased through the baking operation before developing, the portions of the resist layer contacting the layer for etching are difficult to harden and the resist pattern is provided with an undercut shape.
The rounding shape of the head portions and the undercut of the resist pattern which are observed in the chemically sensitizing negative type resist are not so problematic in etching the layer for etching. Further, in principle, the chemically amplified negative type resist utilizes the crosslinking reaction of a resin. Therefore, the thermal resistance thereof is considerable, the mechanical strength thereof is provided with a large value and the deformation of the resist pattern is extremely limited. Accordingly, the chemically amplified negative type resist is superior to the chemically amplified positive type resist in regard with a resist pattern that is employed in hard etching, highly-dosed ion implantation or high energy ion implantation wherein the temperature of the resin is substantially elevated.
However, the following problems have been caused when the chemically amplified negative type resist was coated on the surface of the layer for etching, the resist was prebaked, the KrF excimer laser beam was selectively irradiated on a resist layer comprising the chemically amplified negative type resist, the resist was baked and the resist was developed, to provide a resist pattern of a half .mu.m or less.
Firstly, in the exposure region of the resist layer irradiated with the KrF excimer laser beam, a portion of the film was reduced at the surface of the resist layer, and an encroachment was caused in the portions of the resist layer contacting the layer for etching.
This is because, in the exposure region of the resist layer, the amount of acid from the acid generating agent is reduced by a neutralization reaction caused by a basic substance on the surface of the resist and in the portions of the resist proximate to the surface, and in the portions of the resist contacting the layer for etching, by which the crosslinking density is lowered. As a result, the dissolution rate with respect to the developer immediately before the developing operation on the surface and in the portions proximate to the surface of the resin, and the portions of the resin contacting the layer for etching, becomes faster in comparison with those in the other portions.
Secondly, waviness of the side walls of the developed resist pattern, that is, a ruggedness thereof became conspicuous.
This is due to a fact wherein the dissolution rate in the exposure region of the resist layer is periodically distributed in the depth direction by a standing wave caused by an interference between an incident beam of the KrF excimer laser beam that is incident on the exposure region of the resist layer and a reflecting beam from the surface of the layer for etching.
Thirdly, there caused a variation in the resist film thickness of the resist pattern or a variation in dimensions of the resist.
These are caused by a variation in the reflectance of the surface of the layer for etching.
Fourthly, the difference between the dissolution rates with respect to the developer in the exposure region and the non-exposure region of the resist layer, at the portions of the resist layer contacting the layer for etching, was not made large, and therefore, the resolution was poor.
This is due to a fact wherein, in the exposure region of the resist layer, the dissolution rate with respect to the developer in the portions of the resist layer contacting the layer for etching, is faster than the dissolution rate of the portions of the resist layer on the surface thereof.
As a result, a sufficient resolution and a wide range of depth of focus were not provided, and an excellent shape of the resist pattern was not achieved.